Choose the package you need and Microsoft Windows and Office ISO Download Tool offers further options. We selected Windows 10 Insider Preview, for example, and were presented with a list of 8 recent builds, many of which were available in multiple formats: Client, Enterprise, Education Insider, Core Single Language, China Only, IOT Core.

Click the build you need, select a product language and you're presented with 32-bit (if appropriate) and 64-bit download links. These point to the genuine ISOs on the Microsoft site, not files hosted elsewhere, so it's safe to click and download them as usual.